1. Field of the Invention
The present invention relates to a harmonic generator for obtaining a harmonic by converting a wavelength and a like image display device using the harmonic generator such as a video projector, a television receiver or a liquid crystal panel.
The present invention particularly relates to a harmonic generator for obtaining a high-output harmonic.
2. Description of the Background Art
A nonlinear crystal having a poled structure is used as a laser wavelength conversion element utilizing its nonlinear optical effect. For example, if a laser light having a wavelength of 1064 nm, which is the wavelength of infrared rays, is incident on a nonlinear crystal having a poled structure, a green laser light having a wavelength of 532 nm can be obtained as a second harmonic. Particularly, since the medium of a semiconductor laser for outputting a green wavelength light is yet to be found, the nonlinear crystal having the poled structure can be said to be one of important technologies for obtaining a green laser.
In recent years, image display devices using lasers as light sources have gathered attention. Since a laser light as a coherent light has strong monochromaticity, an image display device having a high color purity and an excellent color reproducibility can be realized by selecting a laser having a suitable wavelength as a light source. Three primary colors of light are red, blue and green. Green lasers are important because green is one of the three primary colors. Accordingly, in order to realize an image display device having higher luminance, improvements in the efficiency and output of a laser are indispensable. Thus, it is expected to realize a high-efficiency and high-output green laser. It also asked for to realize a high-luminance image display and a high-quality image display.
FIG. 23 shows the construction of a harmonic generator for generating a second harmonic using a nonlinear crystal 11 having a poled structure. The harmonic generator is provided with a laser light source 10 for generating a fundamental wave 50, a condenser lens 20 for condensing the fundamental wave 50 and the nonlinear crystal 11 on which the fundamental wave 50 condensed by the condenser lens 20 is incident. The fundamental wave 50 and a second harmonic 51 are emitted from the nonlinear crystal 11. It is known that an output of the second harmonic 51 is proportional to a square of the energy density of the incident laser light (fundamental wave) per unit area in the case of obtaining the second harmonic 51 from the nonlinear crystal 11 having a poled structure. However, the light quantity of the laser light absorbed by the nonlinear crystal 11 increases as the energy density of the laser light in the nonlinear crystal is increased. As a result, temperature increases in parts of the nonlinear crystal 11 where the laser light passes or which is near the focus of the laser light, whereby temperature is locally high at the respective parts in a temperature distribution in the nonlinear crystal 11. Upon a temperature change in the nonlinear crystal 11, the phase matching condition of the nonlinear crystal 11 deviates from a proper one as a refractive index in the nonlinear crystal 11 changes, with the result that a harmonic generation efficiency decreases. Further, if the energy density of the laser light is excessively high, the nonlinear crystal 11 is damaged due to a temperature increase therein.
In a harmonic generator including a laser light source and a nonlinear crystal having a poled structure, the positional relationship of the laser light source and the nonlinear crystal has been conventionally relatively always constant. Thus, if temperature becomes locally high in a temperature distribution in the nonlinear crystal for the above-described reason, a harmonic generation efficiency invariably decreases. As a method for solving this problem, it has been proposed to suppress a local temperature increase in a nonlinear crystal by displacing a laser optical path parallelly to or perpendicularly to an optical axis as disclosed in Japanese Unexamined Patent Publication No. H03-208387. FIGS. 24, 25 and 26 show typical constructions for such a method. FIG. 24 shows the construction for oscillating a laser light passing through a nonlinear crystal 11 by oscillating a scanning mirror 21. A fundamental wave emitted from a laser light source 10 is incident on the nonlinear crystal 11 after being reflected by the scanning mirror 21 and having an incident angle thereof on the nonlinear crystal 11 kept constant by a condenser lens 20. The oscillation of the laser optical path can be realized by oscillating optical elements such as a lens and a mirror or oscillating the nonlinear crystal 11 itself.
FIG. 25 shows the construction for oscillating a laser optical path by rotating a prism 28. A laser light oscillates in a direction perpendicular to an optical axis while having an incident angle thereof on a nonlinear crystal 11 kept constant by passing through the rotating prism 28. FIG. 26 shows the construction for oscillating a nonlinear crystal itself. The nonlinear crystal 11 is mounted on a vertical direction oscillator 24 that oscillates in the direction perpendicular to the optical axis. In the case of oscillating either the laser optical path or the nonlinear crystal 11, resonance (oscillation at a natural frequency) is generally used in consideration of the amplitude of oscillation and an energy to be consumed. By using the resonance, oscillation having a low consumption energy and a large amplitude is possible.
There is also a construction for causing a laser optical path to make a circular movement at a constant speed about an axis parallel to an optical axis as disclosed in Japanese Unexamined Patent Publication No. H07-36072. In the case of a nonlinear crystal having a poled structure, the position of a laser light passing through this crystal is of considerable significance to obtain a high conversion efficiency. Particularly, the displacement of the position of the laser light in the depth direction of the nonlinear crystal has a larger influence on the conversion efficiency than the one in the width direction of the nonlinear crystal. This results from the poling interval of the nonlinear crystal.
Specifically, since the poled structure is grown from a surface in the formation procedure of the nonlinear crystal, the growth of poles are larger toward the surface to decrease the poling interval as shown in FIG. 27. On the contrary, the more distant from the surface of the nonlinear crystal, the larger the poling interval. In the case of obtaining a harmonic from the nonlinear crystal, a suitable poling interval is necessary, but an irradiation range of the laser light for obtaining the suitable interval has a limited width in the depth direction of the nonlinear crystal. Thus, if an actual irradiated position of the laser light largely deviates from the irradiation range for obtaining the suitable interval, the harmonic generation efficiency decreases. This is described in detail with reference to FIG. 28 showing a relationship between the displacement of the irradiated position of the laser light in the depth direction and the harmonic conversion efficiency. If yl denotes the maximum conversion efficiency of the laser light, it is necessary to irradiate the laser light within a range of ±100 μm in the depth direction from an optimal irradiated position x1 of the laser light in order to obtain an efficiency of 90% (0.9×y1) or higher of the maximum conversion efficiency y1. Further, in order to obtain an efficiency of 50% (0.5×y1) or higher of the maximum conversion efficiency y1, it is necessary to irradiate the laser light within a range of ±250 μm in the depth direction from the optimal irradiated position x1 of the laser light.
On the other hand, even if the irradiated position of the laser light is displaced in the width direction (see FIG. 27) of the nonlinear crystal, the poling interval in the irradiation range of the laser light does not largely vary, wherefore the distance of the irradiated position of the laser light in the width direction has a little influence on the conversion efficiency. Since we use a nonlinear crystal with a poled structure having a width of 1 mm (inclusive) to 26 mm (inclusive), a ratio of the width to the depth of the irradiation range of the laser light to obtain a conversion efficiency of 90% or higher is 5:1 to 130:1. In other words, a permissible displacement range of the irradiated position of the laser light in the depth direction of the nonlinear crystal is significantly smaller than the one in the width direction of the nonlinear crystal. In the case of causing the laser optical path to make a circular motion, the irradiated position is displaced by the same amount both in the depth direction and in the width direction of the element. Accordingly, an increase in the displacement of the laser optical path in the construction adopting the circular motion leads to an increase of the displacement in the depth direction to vary the poling interval, thereby causing a problem of reducing the conversion efficiency. This point was not considered in the patent literature 2.
There has been also proposed a method for adjusting an energy density by displacing the focus position of a laser light in an element as disclosed in Japanese Unexamined Patent Publication No. 2006-267377. Since a second harmonic outputted from a nonlinear crystal is proportional to the square of the energy density of a fundamental wave per unit area, the laser light of the fundamental wave is generally incident on the nonlinear crystal by being condensed by a condenser lens. As shown in FIG. 30, a nonlinear crystal 11 is mounted on an optical-axis direction oscillator 22 movable in an optical axis direction. A device disclosed in this publication is constructed to prevent the damage of an element by the laser light by adjusting the focus position in the optical axis direction in accordance with the energy density of a light source for emitting the fundamental wave.
On the other hand, if a laser light is used as a light source for an image display device to realize the image display with higher quality, this leads to a problem that an interference pattern peculiar to laser called a speckle noise occurs. Specifically, since the laser light has a narrow spectral width and high coherency, the scattered laser lights randomly interfere with each other to cause a speckle noise in the form of fine particles. As a method for reducing the speckle noise, there are generally known a method for swinging a passing laser light by swinging an optical element such as a diffuser plate and a method for swinging a screen on which a video is projected. If either one of these methods is used, the phase distribution of light on the screen varies and the fine pattern of the speckle noise also changes with time. If the pattern of the speckle noise changes faster than an afterimage time of an observer, the observer senses as a noise-free image since the speckle noise is time-averaged by the eyes of the observer.
The above-described conventional method for displacing the laser light intends to prevent such a phenomenon that the nonlinear crystal absorbs the laser light, thereby causing a temperature increase of the nonlinear crystal to damage the nonlinear crystal.
However, with the higher efficiency and higher output of the laser light, the damage of the nonlinear crystal cannot be sufficiently prevented by the conventional method, which has caused a problem that no stable laser light can be obtained.
On the other hand, in order to realize an image display device with a high image quality, the speckle noise is reduced by displacing the phase of the laser light through the movement of the optical element or by displacing the phase of the laser light through the movement of the projection screen. However, since the optical element and the screen generally reduce the noise through the resonance, noise generation and the like have been problematic depending on the amplitude of the resonance.